Magneto caloric material (MCM)—i.e. a material that exhibits the magneto caloric effect—provides a potential alternative to fluid refrigerants used in e.g., heat pump applications. In general, the magnetic moments of a normal MCM will become more ordered under an increasing, externally applied magnetic field and cause the MCM to generate heat. Conversely, decreasing the externally applied magnetic field will allow the magnetic moments of the MCM to become more disordered and allow the MCM to absorb heat. Some MCM types exhibit the opposite behavior—i.e. generating heat when a magnetic field is removed and becoming cooler when placed into the magnetic field. This latter type can be referred to as inverse or para-magneto caloric material. Both normal and inverse MCM are referred to collectively herein as magneto caloric material or MCM.
The theoretical cycle efficiency of a refrigeration cycle based on an MCM and the magnetic caloric effect can be significantly higher than for a comparable refrigeration cycle based on a fluid refrigerant. However, challenges exist for the practical and cost competitive use of an MCM. In addition to the development of suitable types of MCM, equipment that can attractively utilize an MCM is still needed.
One such challenge relates to the magnets used to provide magnetic flux for acting upon the MCM. Rare earth magnets that can provide the level of magnetic flux required are relatively expensive and can account for as much as 50 percent of the cost of an MCM-based heat pump. Additionally, the cost of rare earth magnets can be volatile, which in turn can affect the price stability of MCM-based heat pump equipment such as e.g., refrigerator appliances and air-conditioning systems. The manufacture of magnets into specific shapes as may be required depending upon the heat pump design can also significantly increase their cost.
Increasing the field strength provided by the magnet(s) used with an MCM-based heat pump can improve the magnetic caloric response of the MCM. Increasing the volume of the magnetic field may also improve efficiency of the MCM-based heat pump and a minimum magnetic field strength may be needed to achieve optimum performance. Unfortunately, the size of the magnet increases quadratically with increases in the required field strength and size. Further, where iron is used to help shape the magnetic field, such can undesirably increase the weight of the MCM-based heat pump.
Accordingly, a magnetic device for a magnetocaloric (i.e. MCM-based) heat pump would be useful. More particularly, a magnetic device that can provide high magnetic field strength where needed while optimizing the amount of rare earth magnets required would be beneficial. Such a device that can also optimize the amount of iron or other materials used to form the magnetic field would also be useful.